The present invention relates to a process of producing doubly oriented fibres and films of aliphatic polyamides. The fibres and films of the present invention are characterized by the fact that they exhibit a higher tensile strength than that of the doubly oriented fibres and films of aliphatic polyamides prepared according to the conventional techniques.
It is known that the tensile strength or modulus of elasticity (M.E.) of fibres formed of linear polymers, in particular, polyamides, is considerably lower than that which could be theoretically expected, if the macromolecules would be elongated and oriented parallelely to the fibre axis. For instance, said modulus (M.E.) of a typical fibre of nylon 6, prepared according to the conventional spinning processes is of the order of 200 kg/mm.sup.2, while the value which can be theoretically obtained is at least of the order of a higher magnitude. This difference can be attributed to the fact that in the conventional spinning processes the complete parallelization and elongation of the single macromolecular chains cannot be attained along the fibre axis, but, on the contrary, an only partially elongated structure is obtained oriented along the axis of the fibre. If in the process of preparation of the fibre it would be possible to elongate and orient all the macromolecules along the fibre axis, the fibre itself should attain the maximum value of the M.E. which could be theorically expected on the basis of the stresses which are necessary for the deformation of the chemical linkages formed along the macromolecular chain.
Attempts have been recently made to obtain unusual mechanical properties in the polymeric solids. According to the U.S. Pat. No. 3,671,542 of the E. I. du Pont de Nemours and Company, aromatic polyamides have been obtained, which exibit a modulus M.E. up to about 9000 kg/mm.sup.2 (equivalent to about 1000 g/denier) by a wet spinning process, starting from anisotropic solutions. The possibility of preparing anisotropic solutions is however limited to polymers in which the presence of an aromatic ring assures a rigid structure of the single molecules dispersed in the solution. In the case of non-aromatic polyamides, such as those formed by polycondensation of Epsilon-caprolactam (nylon 6) and pyrrolidone (nylon 4), which cannot form anisotropic solutions, the method disclosed in the aforementioned U.S. Pat. No. 3,671,542 cannot be applied.
Another approach aiming to provide unusual mechanical properties in the polymeric solids has been disclosed in the British Patent Application No. 10746/73 in the name of G. Capaccio and I. M. Ward.
In this British patent application provision has been made to prepare a polyethylene having a modulus of elasticity which can reach the aforementioned value which has been reached by the aromatic polyamides by applying cold extrusion and by the control of the molecular weight. However, this method does not relate to polyamides.